1. Field of the Invention
The present invention relates to an exchange coupling film comprising, from the bottom to the top, a seed layer, an antiferromagnetic layer and a ferromagnetic layer in which the magnetization direction of the ferromagnetic layer is aligned in a given direction due to an exchange coupling magnetic field generated at the interface between the antiferromagnetic layer and ferromagnetic layer, and to a magnetic sensing element (such as a spin-valve type thin film element and an AMR element) using the exchange coupling film. In particular, the present invention relates to an exchange coupling film capable of more properly improving electromigration resistance in high density recording in the future as compared with that in the related art, while enabling a favorable rate of change of resistance to be obtained, and to a magnetic sensing element using the exchange coupling film.
2. Description of the Related Art
FIG. 15 shows a partial cross section of a conventional magnetic sensing element (spin-valve type thin film element) cut from a direction parallel to an opposed face to a recording medium.
The reference numeral 14 shown in FIG. 15 denotes a seed layer made of, for example, a NiFeCr alloy. An antiferromagnetic layer 30, a pinned magnetic layer 31, a nonmagnetic layer 32, a free magnetic layer 33 and a protective layer 7 are sequentially laminated on the seed layer 14.
An exchange coupling magnetic field is generated in this type of the spin-valve type thin film element at an interface between the antiferromagnetic layer 30 and pinned magnetic layer 31 by a heat treatment, and magnetization of the pinned magnetic layer 31 is fixed in the height direction (the Y-direction in the drawing).
Hard bias layers 5 are formed at both sides of the multilayer from the seed layer 14 to the protective layer 7 as shown in FIG. 15. Magnetization of the free magnetic layer 33 is aligned in the track width direction (the X-direction in the drawing) by a longitudinal bias magnetic field from the hard bias layer 5.
As shown in FIG. 15, an electrode layer 8 is formed so as to cover the hard bias layer 5. A sense current from the electrode layer 8 mainly flows through three layers of the pinned magnetic layer 31, non-magnetic layer 32 and free magnetic layer 33.
The seed layer 14 is formed under the antiferromagnetic layer 30 in the spin-valve type thin film element shown in FIG. 15, because improvements of electromigration resistance represented by the electromigration resistance and the rate of, change of resistivity could be expected by providing the seed layer 14.
It has been emphasized in the related art that the crystal structure of the seed layer 14 is a face-centered cubic structure (a fcc structure).
Layers formed on the seed layer 14 may be properly oriented in the [111] direction and the crystal grain size may be increased when the seed layer 14 takes the face-centered cubic structure. As a result, electric conductance may be increased while increasing the exchange coupling magnetic field generated between the pinned magnetic layer 31 and antiferromagnetic layer 30 to enable improved electromigration resistance to be expected.
The seed layer 14 has been formed with the NiFeCr alloy with a Cr composition ratio of 40 at %, in order to maintain the crystal structure of the seed layer 14 as the face-centered cubic structure.
However, the sense current density flowing through the spin-valve type thin film element has increased as the spin-valve type thin film element is compacted for complying with high density recording in the future, thereby causing a problem of electromigration.
The inventors of the present invention have thought it important to contemplate improving wettability of the surface of the seed layer 14 in contact with the antiferromagnetic layer 30 for solving the problems as hitherto described. It was conjectured that each atom in an antiferromagnetic material constituting the antiferromagnetic layer 30 may be hardly coagulated on the seed layer 14 having good wettability when the antiferromagnetic layer 30 is deposited on the seed layer 14 by sputtering, which results in a large crystal grain diameter to enable the exchange coupling magnetic field generated between the antiferromagnetic layer 30 and pinned magnetic layer 31 and the rate of change of resistance to be increased.
While the larger the composition ratio of Cr incorporated in the seed layer 14 is supposed to be better for improving wettability, increasing the composition ratio of Cr too much may result in precipitation of a body-centered cubic structure (bcc structure) mingled with the face-centered cubic structure (fcc structure) with less improvement of wettability than expected. Consequently, the exchange coupling magnetic field generated between the pinned magnetic layer 31 and antiferromagnetic layer 30 rather decreases to make it impossible to improve electromigration resistance, which is represented by electron migration resistance, as well as the rate of change of resistance.
Electromigration resistance and the rate of change of resistance were forced to be decreased unless the composition ratio of Cr is adjusted to be 40 at % or less, when the seed layer 14 is formed of the NiFeCr alloy as in the related art. Furthermore, it was revealed that using the NiFeCr alloy for the seed layer 14 makes it difficult to contemplate obtaining a higher exchange coupling magnetic field for further improving electromigration resistance and the rate of change of resistance in compliance with high recording density in the future.
In addition, using the NiFeCr alloy (with a Cr composition ratio of 40 at % or less) in the seed layer 14 have caused the following problems because lubricity of the surface of the antiferromagnetic layer 30 decreases due to undulations generated between crystal grain boundaries on the surface of the antiferromagnetic layer 30.
FIG. 16 illustrates a partially magnified drawing of the structure of the magnetic sensing element shown in FIG. 15. The seed layer 14 shown in FIG. 16 is formed of the NiFeCr alloy with a Cr composition ratio of 40 at % or less.
FIG. 16 shows the surface 30a having undulations between the crystal grain boundaries formed on the antiferromagnetic layer 30. These undulations also appear on the surfaces of the pinned magnetic layer 31, nonmagnetic layer 32 and free magnetic layer 33 formed on the antiferromagnetic layer 30.
Magnetic poles are generated at undulated portions on the surface of the pinned magnetic layer 31 as shown in FIG. 17 (a schematic cross section obtained by cutting the pinned magnetic layer 31, nonmagnetic layer 32 and free magnetic layer 33 in the Y-direction shown in FIG. 16) by generation of undulations as described above. The magnetic poles also appear at undulated portions of the free magnetic layer 33 opposed to the pinned magnetic layer 31 with intervention of the nonmagnetic layer 32, thereby increasing an interlayer coupling magnetic field (Hin) caused by a static magnetic coupling (topological coupling) between the pinned magnetic layer 31 and free magnetic layer 33. Accordingly, an action for magnetizing in the Y direction is exerted on the free magnetic layer 33 that should be naturally magnetized in the X-direction, causing a problem that asymmetry in the regenerative waveform is increased.
When a specular reflection layer is formed on the free magnetic layer 33 formed of, for example, an oxide of Ta, on the other hand, lubricity of the surface of the specular reflection layer is also inhibited by being affected by the undulation of the surface 30a of the antiferromagnetic layer 30 to decrease specular reflectivity of the specular reflection layer, making it impossible to expect increased rate of change of resistance caused by the specular effect.
The inventors of the present invention have presumed that the problems arising from surface undulation on the surface of each layer formed on the seed layer 14 comes from poor wettability of the seed layer 14 and large crystal grains formed on the seed layer 14.